STRUCTURE OF SOME ANGLESEY ROCKS. 



233 



rather irregular outline in a finely granular and rather earthy-looking 

 matrix — most probably the result, in the main, of the decomposition 

 of rather imperfectly crystallized felspar, -which consists now of 

 an admixture of earthy dust, doubly refracting clear microliths, 

 and minute scales of mica or chlorite. The quartz grains show 

 an indistinct banded order, and contain many minute cavities. The 

 rock appears to have been brecciated more than once in situ. The 

 older cracks are mainly filled up by quartz, the newer by calcite 

 with a little chlorite. In one part of the slide the faulting of an 

 older vein and subsequent infiltration of a newer one is very 

 clearly shown. Microscopically the rock has a resemblance to the 

 Treffgarn halleflinta, but under the microscope it is more gneissic. 



26. (Gwalchmai, p. 218.) Is certainly of fragmental origin, show- 

 ing stratification foliation. The quartz grains have fewer inclusions ; 

 there is felspar much as before, and a fair quantity of mica ; most of 

 this is of a very pale golden-yellow colour, is feebly dichroic, and 

 gives brilliant tints between the jSTicols; the rest is dull green, 

 probably an alteration produced after biotite. Some granular matter 

 as above. 



27. (N.W. of Gwalchmai, p. 218.) More resembles 25, but is more 

 compact. I think that it, too, is of fragmental origin, and is pro- 

 bably the result of the metamorphosis of a fine quartzose silt. 



28. (N.W. of Gwalchmai, p. 218.) Is certainly fragmental, con- 

 sisting of quartz with a fair amount of chlorite, some white mica, 

 and a few grains (probably fragmental) of a closely twinned plagio- 

 clastic felspar. 



29. (Summit of Bodafon Mountain, p. 219.) Is a quartz-schist 

 with a fair amount of ferrite and minute chlorite (?). 



31. (Holyhead Mountain, p. 226.) Also a quartz-schist with a fair 

 amount of a pale-green mica, as above, the ground-mass consisting of 

 this mineral with agglutinated very minute grains of quartz. In this 

 are scattered quartz grains of larger size up to about 0"-03 diameter, 

 clearly of fragmental origin, some having a secondary deposit of 

 quartz on their edges. Minute cavities are common in these grains, 

 and one or two contain some dark hair-like microliths, a grain or two 

 of epidote, and possibly one of tourmaline. 



32. (Khoscolyn church, p. 227.) Also a quartz-schist resembling 

 the last, but more uniform in structure; the same adventitious 

 minerals as in the last. 



33. (S. of Borth Saint, p. 227.) Another quartz-schist with rather 

 more mica and chlorite, a few grains of felspar, and the same adven- 

 titious minerals. jSTumerous grains of a granular earthy mineral 

 (? an epidote). 



34. (Cutting E. of Ty Croes, p. 217.) Quartz and a white mica 

 are the most conspicuous minerals, with an occasional grain of 

 felspar ; but a sort of granular paste in which these are imbedded, 

 consisting of microliths of white mica and other minerals, may in 

 some cases have replaced a felspar constituent. The rock is much 

 altered, but, I suspect, still retains in its large grains traces of its 

 original fragmental structure. 



